1. Field of the Invention
This invention relates generally to temperature control of devices by primarily passive mechanisms, for example as may be used to adjust the wavelength of a semiconductor laser by changing its temperature.
2. Background and Relevant Art
Semiconductor lasers have been tuned in wavelength through controlled variation in temperature. Two common means of changing the temperature are thermoelectric coolers and resistive heaters. Thermoelectric coolers typically use the Peltier effect. This device can either heat or cool depending on the direction of the electrical current flowing through the Peltier elements. A resistive heater is a resistor which converts electrical current to heat.
In either case, the method of temperature variation consumes considerable electrical power. In the case of a thermoelectric cooler, the Peltier effect has limited efficiency and the electric power consumption required for cooling is typically several times the power consumed in the laser. When the thermoelectric cooler is used as a heater, it is approximately as efficient as a resistor. While heating can then be more power efficient than cooling, it is still necessary to supply significant electrical power. This is so because it is in general not desirable to operate the laser at high temperatures. To avoid such operation, the laser is generally mounted in such a way that the thermal impedance to an appropriate heat sink is small. In consequence, to obtain a significant additional temperature rise from a heater which is located in proximity to the laser, it is necessary to generate heat of an order of magnitude that is significantly greater than is generated by the laser. If the heater is located downstream of the laser with respect to the heat flow, the situation is even worse, for in that case the thermal impedance to the heat sink is even smaller and more heat must be generated to effect the same temperature rise.
The additional power that must be provided for temperature tuning may not be acceptable in situations where power consumption is important, for example if the device is operated from a modest sized battery for a significant length of time.
Besides, the issue of power consumption, it may not be physically convenient to deploy the heater in proximity to the laser. If cost is a serious consideration and high volume standard parts such as Compact Disk (CD) lasers are desirable to be used, it may be necessary to break open the packaging to add the heater. Thus, there is a need for approaches that would enable the laser to be temperature tuned, without disturbing the packaging in which it may be readily purchased.
U.S. Pat. No. 5,371,753 discloses an apparatus in which the thermal impedance of a laser diode heat sink varies during the turn-on cycle of the laser. The variation is accomplished through use of a metal structure and an air-gap, which metal structure by virtue of its dimensions is open while the laser is warming up, and closes when the temperature has reached the range of desired values. The closure is accomplished by virtue of reversible deformations arising from temperature changes such as may be obtained with shape-memory alloy metals. While this approach can allow the laser to reach a specific temperature range, there is no means by which the temperature that is ultimately reached can be varied significantly or controlled precisely. The thermal impedance is not adjustable during device operation. It is fixed during fabrication of the device and this, in turn, fixes the temperature range.
U.S. Pat. No. 6,243,404 discloses a laser module that can be tuned over a temperature range and different temperature ranges can be selected during fabrication. The temperature range is selected by inserting spacers of known thermal impedance between the laser and the ultimate heat sink such that the laser temperature can be made to rise by a fixed known amount above ambient. Then, a secondary control mechanism such as a thermoelectric cooler can adjust the laser temperature over a range in the vicinity of the base laser temperature. In this approach, the thermal impedance (and therefore the base laser temperature) is selected at the time of device fabrication and is afterwards fixed. The temperature cannot then later be further adjusted through variations in the thermal impedance and this approach depends on previously described cooling or heating schemes, with their consequent inefficiencies, to accomplish the required temperature adjustments during device operation.
Thus, there is a need to control the temperature of devices, such as semiconductor lasers, in a manner that consumes less energy than current approaches.